What it shows
A wire carrying current in a magnetic field feels a sideways push, F = B I L. Reversing the current or the magnet reverses the push, as Fleming’s left-hand rule says.
The science
A magnetic field B exerts a force F = B I L sin θ on a length L of wire carrying current I, at right angles to both the field and the wire. The direction is given by Fleming’s left-hand rule. For a fixed field and length, F is proportional to I, so the sideways swing grows as the current grows. This force on a conductor is what turns every electric motor.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two ceramic ferrite magnets from an old loudspeaker, placed on a steel strip or an iron bar to make a gap
- A strip of kitchen aluminium foil, 1 cm wide and 15 cm long, folded to a stiff ribbon
- Two cells (3 V) in a holder and a rheostat or 10 ohm resistor
- A wooden frame or a cardboard box with two paper clips as hangers
- A protractor and a ruler
- Flexible wire, a switch and crocodile clips
Safety
For this project: An adult must supervise: the foil can get warm and the current is large for a short time, so tap the switch only for a second or two. Keep magnets away from phones, watches and bank cards.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the magnets on the steel bar with the north pole of one facing the south pole of the other, leaving a gap of about 2 cm; the field between them is then fairly uniform.
Hang the foil strip from the two paper clips so that its lower part hangs through the middle of the gap, free to swing like a pendulum.
Join the foil to the cells, switch and resistor so that the current flows along the strip, and tap the switch briefly.
Note the direction of the swing, then reverse the cell connections and note that the swing reverses.
Turn one magnet round so that the field reverses and check the swing direction against Fleming’s left-hand rule.
Change the resistor to give different currents, measure each current with a multimeter, and record the maximum swing angle for each.
Plot the swing angle against current and see that it increases steadily with the current.
What you should see
The foil jumps sideways when the switch is tapped and the swing reverses with the current or with the field. The swing angle grows with the current, showing the force is proportional to I.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the formula for the force on a current-carrying wire?
F = B I L sin θ, where θ is the angle between the wire and the field.
2What is Fleming’s left-hand rule used for?
To find the direction of the force: forefinger for the field, middle finger for the current, thumb for the force.
3When is the force zero?
When the wire is parallel to the field, because sin θ is then zero.
4Which machine uses this effect?
The electric motor, and also the moving-coil meter and loudspeaker.
Take it further
Estimate the field B between the magnets from F = B I L using the swing angle and the mass of the foil strip.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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